There's people around here who are far more qualified to answer your questions, but when you're filtering (frequencies, anyway) you want to make some frequencies terribly apparent and totally omit others. If you've got some type of simulation program, like spice, simulate a simple filter circuit and analyze it v. frequency. You'll see its quite far from the total-on, total-off result you want. In fact, the response is sinusoidal, and there's no clear cutoff between accentuated frequencies and subdued ones.

Staff: Mentor

To get more than a couple of poles and zeros (to increase the difference in gain between the passband and stopbands of your filter), you need to use active filters. There are three main topologies for active filters -- Butterworth, Cheby and Elliptical. Each topology has its strong and weak points, so you will choose which one to implement based on the requirements of your filter.

Iductors are generally not used in audio filters because of the physical size required. You can emulate an inductor with an active circuit called a "gyrator", so inductors are really not needed for most audio filter applications. The exception might be power audio applications, like audio speakers and crossover networks.

Staff: Mentor

electronic engineer said:

I'm sorry , but i really can't understand that point , could you clarify more please?!

thanks a lot!

If possible, I'd recommend that you get a copy of "The Art of Electronics" by Horowitz and Hill. It's an excellent introductory electronics book, with a little theory and lots of practical insights. Chapter 4 takes you from simple RC filters into active filters, and explains the tradeoffs between the different types.

Electronic Engineer: Perhaps you could give us a snapshot of your current education?

sure, in fact i'm now studying electronic engineering 4'th year still undergraduated ...for me the problem is that our course is very weak , i have to absolutely depend on myself that's why i'm intending this wonderful forum to get answers and some guidance.
some of posts came out from our electronics labs , because i think that we take so strong practical and theoretical info comparing to our theoretical and practicall background...
I don't know exactly in which way the electronic engineer have to behave to make himself as strong base in electronics, is there any initial advise?!

I might not say many things , thanks for such interest and we could discuss more if you don't mind!

in a word, unless you use both inductors and capicitors, you cannot get the Q needed for interesting filters if your components are just passive Rs and Cs. you need gain and feedback to get some decent Q factors.